1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of manufacturing the same, and more particularly to a liquid crystal display device for use in miniature portable apparatuses such as a cellular phone and an electronic notebook, and a method of manufacturing the same.
2. Prior Art
In recent years, with rapid development of information communication technology, liquid crystal display devices have acquired an increasing importance, not least in the field of miniature portable apparatuses such as a cellular phone and an electronic notebook.
A liquid crystal display device of this kind is comprised of a liquid crystal cell formed by encapsulating liquid crystal in an airtight manner between two glass substrates (first and second glass substrates), and a driving circuit (hereinafter referred to as xe2x80x9cdriving ICxe2x80x9d) arranged on the periphery of the liquid crystal cell, for driving and controlling the liquid crystal cell, the liquid crystal cell and the driving IC together composing a single module.
Conventionally, in the manufacture of a miniature module, a so-called xe2x80x9cmulti-cell bondingxe2x80x9d method has been employed in which a multiplicity of liquid crystal cells are formed on a large-sized glass substrate in a single continuous process by carrying out a multi-cell bonding. Subsequently, driving ICs are directly mounted onto a liquid crystal panel by means of a so-called COG (chip on glass) mounting method. This manufacturing method allows a multiplicity of liquid crystal cells to be obtained in a single process by means of multi-cell bonding, leading to a substantial improvement of productivity, which, in turn, is combined with the adoption of the COG mounting method to reduce the number of manufacturing steps, and allow the manufacture of miniature modules at low costs.
FIG. 16 is a top plan view schematically showing essential parts of a conventional liquid crystal display device of this kind. Specifically, the liquid crystal display device includes a seal 52 in the form of a frame having a liquid crystal injection port 51 and sandwiched between a lower first glass substrate 53 and an upper second glass substrate 54, both transparent. The injection port 51, upon injection of liquid crystal 55 therethrough into the interior of the seal, is sealed with a sealant such as an ultraviolet ray-setting resin (hereinafter referred to as xe2x80x9cUV-setting resinxe2x80x9d) or the like, thereby hermetically encapsulating the liquid crystal 55 with the seal 52 in an airtight manner. A vertical conducting portion 56 is provided in the vicinity of a corner of the seal 52, for conductive connection between respective transparent conductive films formed on the surfaces of the first and second glass substrates. The seal 52 is formed by screen-printing in a frame pattern a sealant which contains epoxy resin as a primary constituent.
FIG. 17 is a sectional view showing details of the seal 52. As shown in the figure, the seal 52 contains a multiplicity of spacers 58 and conductive particles 59 mixed and generally evenly dispersed therein. The thickness of the seal 52 is controlled by the spacers 58, and the conductive connection between the transparent conductive films formed on the surfaces of the first and second glass substrates 53, 54 is ensured by means of anisotropic conduction obtained with the conductive particles 59. Although the conductive particles 59 has only to be mixed into the vertical conducting portion 56 alone in order to secure conductive connection between the transparent conductive films, they are generally mixed in the seal 52 uniformly over the entire region thereof in view of production efficiency.
In the modern information society, increasingly refined display patterns on the liquid crystal display are demanded so as to be able to display as much information as possible. To this end, connection wiring 57 connected to the driving IC (see FIG. 16) needs to be formed with a narrow line width, especially in the vicinity of the driving IC, where the connection wiring 57 needs to be formed extremely fine with a line width not exceeding 100 xcexcm.
However, when the connection wiring 57 is formed extremely fine, the wiring resistance becomes so high that if the connection wiring 57 is composed solely of the transparent conductive film, a voltage drop in the connection wiring would become so large as to interfere with the normal operation of the liquid crystal display.
Therefore, it has been a conventional practice to deposit a metal thin film which has small wiring resistance and hence excellent conductivity on the transparent conductive film such that the connection wiring 57 has a two-layered structure composed of a transparent conductive film and a metal thin film.
As described above, the first and second glass substrates 53, 54 have the transparent conductive film, specifically an indium tin oxide (hereinafter referred to as xe2x80x9cITOxe2x80x9d) film, formed on respective surfaces thereof. A transparent insulating film is further laminated on the surface of the ITO film at a portion corresponding to the liquid crystal display portion. Conventionally, this transparent insulating film is used as a mask to perform electroless nickel (Ni)xe2x80x94phosphorus (P) alloy plating and electroless gold (Au) plating on the surface of the transparent conductive film, thereby forming a metal thin film on a specified portion of the ITO film. Desired extremely fine connection wiring 57 can be obtained in this way while preventing the metal thin film from adhering to the liquid crystal display portion.
In the electroless plating, a Nixe2x80x94P alloy precipitates selectively on the portion where the ITO film is formed, and then Au precipitates by substitution reaction with P on the Nixe2x80x94P alloy as well as by an autocatalytic reaction of Au ion. Thus, desired extremely fine connection wiring 57 can be obtained without the plated film adhering to the liquid crystal display portion. In this way, a metal precipitates selectively on the ITO film as a transparent conductive film, depositing a metal thin film on the ITO film. Since a metal film is not formed on those portions where the ITO film is not formed, connection wiring 57 composed of a metal thin film can be formed at the desired portion without need for patterning to form the metal thin film.
However, in the conventional liquid crystal display device, as described above, electroless plating is performed using the transparent insulating film as a mask so that the plated film, that is, the metal thin film, deposits also on the transparent conductive film in the vertical conducting portion 56 where no transparent insulating film is coated.
Further, since a multiplicity of liquid crystal cells are, as described earlier, produced at one time by xe2x80x9cmulti-cell bondingxe2x80x9d in a single continuous process, the metal thin film deposits in the vertical conducting portion 56 on the side of the second glass substrate 54 as well as on the side of the first glass substrate 53.
FIG. 18 is a sectional view taken along line Xxe2x80x94X in FIG. 16, and FIG. 19 is a sectional view taken along line XIxe2x80x94XI in FIG. 18. ITO films 60, 61 are formed on the opposed surfaces of the first and second glass substrates 53, 54, respectively. Since no transparent insulating film is formed on the vertical conducting portion 56 on the first and second glass substrates 53, 54, the plating treatment is also performed on portions of the ITO film 60, 61 of the vertical conducting portion 56 to form metal thin films 62.
Therefore, if the spacers 58 used in the vertical conducting portion 56 have the same particle diameter as those in portions other than the vertical conducting portion 56, the thickness of the seal 52 in the vertical conducting portion 56 becomes larger than that in the other portions by the combined film thickness 2xc3x97txe2x80x2 of both metal films 62 on the opposed surfaces of the first and second glass substrates 53, 54, txe2x80x2 representing the thickness of the metal film 62. This gives rise to a variation of the thickness of the seal 52, leading to non-uniform distribution of the thickness of the liquid crystal layer, resulting in unevenness in the background color during liquid crystal display.
As described above, the seal 52 is formed by screen-printing in a frame pattern a sealant containing epoxy resin as a primary constituent. This gives rise to another problem that resin thin films 64 are formed on the upper and lower surfaces of the seal 52, and these resin thin films 64 act as insulating films, leading to conduction failure. That is, as shown in FIG. 20 by way of example, the first glass substrate 53 has the ITO film 61 and metal thin film 62 laminated on the upper surface thereof, and the resin thin film 64 is formed on the lower surface of the seal 52. A similar resin thin film 64 is likewise formed on the upper surface of the seal 52. These resin thin films 64 may block the electrical conduction between the ITO films 60, 61 and the conductive particle 59, and may lead to conduction failure.
It is a first object of the present invention to provide a liquid crystal display device and a method of manufacturing the same, which do not give rise to color unevenness and exhibits improved reliability.
It is a second object of the present invention to provide a liquid crystal display device and a method of manufacturing the same, which do not give rise to color unevenness nor conduction failure, and exhibits improved reliability.
Since the conducting portion (vertical conducting portion) provided in the seal is for securing conductive connection between the transparent conductive films (ITO films), the metal thin films on the transparent conductive films are considered unnecessary in the conducting portion.
When plating treatment is performed, however, metal thin films necessarily deposit on the transparent conductive films in the conducting portion, because no transparent insulating film has been coated on the conducting portion.
On the other hand, in view of the production efficiency of liquid crystal display devices, it is considered necessary that liquid crystal cells be produced with high efficiency by performing multi-cell bonding on a large-sized glass substrate as is employed in the prior art.
Therefore, in view of the present state of production technology and operational efficiency, it is considered desirable to deal with the problem of color unevenness in the background color during liquid crystal display on the assumption that the metal thin films necessarily deposit on the transparent conductive films.
Thus, the present inventor has made intensive studies in order to provide a liquid crystal display device that can achieve the first object on the assumption that the metal thin films deposit on the transparent conductive films in the conducting portion, and as a result, reached the finding that the thickness variation of the liquid crystal layer needs to be limited to not greater than 0.1 xcexcm in order to reduce the color unevenness to a level as low as possible and hence obtain uniform background color. Further, in order to restrain the thickness variation of the liquid crystal layer to not greater than 0.1 xcexcm, it has been found necessary that first spacer members mixed in the conducting portion be formed smaller in size than second spacer members mixed in other portions of the seal by a predetermined value X so as to reduce the thickness of the seal and thereby make the thickness distribution of the liquid crystal layer generally uniform.
The present invention is based on the above described finding. Thus, a liquid crystal display device according to the present invention includes a first transparent substrate having a first transparent conductive film formed on a surface thereof, a second transparent substrate having a second transparent conductive film formed on a surface thereof opposed to the first transparent conductive film, liquid crystal sandwiched between the first transparent substrate and the second transparent substrate, a seal encapsulating the liquid crystal in an airtight manner, and a driving circuit, the seal including a conducting portion (conducting seal portion) disposed so as to electrically interconnect the first transparent conductive film and the second transparent conductive film, the first and second transparent conductive films having metal thin films deposited on respective predetermined surface portions thereof including the conducting portion, the driving circuit being connected to the metal thin films, for carrying out liquid crystal display control.
The liquid crystal display device according to the present invention is characterized by an improvement comprising a plurality of spacer members mixed and generally evenly dispersed in the seal, for controlling the thickness of the seal, the plurality of spacer members comprising first spacer members mixed in the conducting portion, and second spacer members mixed in portions of the seal other than the conducting portion, the first spacer members having a particle diameter smaller than that of the second spacer members by a predetermined value X.
With the above described construction, the first spacer members mixed in the conducting portion have a diameter smaller than that of the second spacer members mixed in a portion of the seal other than the conducting portion by the predetermined value X. As a result, even if metal thin films are deposited on the transparent conductive films, the thickness of the seal at the conducting portion can be kept small, and the thickness distribution of the liquid crystal layer can be made generally uniform.
Further, it has been found by the present inventor as a result of further intensive studies that the predetermined value X is dependent upon the film thickness of the metal thin films. Specifically, it has been found that by setting the predetermined value X in a range of (2t+0.5) to (2txe2x88x920.6) xcexcm, the thickness of the seal can be made generally uniform so that color unevenness of the background color during liquid crystal display can be eliminated.
Thus, the present invention is also characterized in that the predetermined value X is set in a range of (2t+0.5) to (2txe2x88x920.6) xcexcm (t represents the film thickness of the metal thin films).
Further, after intensive studies to provide a liquid crystal display device that can achieve the second object of the present invention, it has been found by the present inventor that occurrence of conduction failure can be avoided by forming a multiplicity of protrusions on the metal thin films, that have a height exceeding the film thickness of the resin thin film.
Thus, a liquid crystal display device according to the present invention is characterized in that, besides a multiplicity of conductive members being mixed in the conducting portion, a multiplicity of protrusions are formed on the metal thin films, and the first and second transparent conductive films are electrically interconnected via the conductive members and the protrusions.
Further, it has been found from the studies conducted by the present inventor that the film thickness of the resin thin films formed on the upper and lower surfaces of the seal is not greater than 0.04 xcexcm. Therefore, the peak height of the protrusions needs to be at least larger than the thickness of the resin thin films. On the other hand, if the peak height of the protrusions is too large, it is difficult to control the thickness of the seal. Further, if the number of the protrusions per 1 xcexcm2 is too small, that is, if the number density of the protrusions is too small, the conductive members and the protrusions would not come into contact with each other. But, if the number density is too large, adjoining protrusions may be merged together. Thus, there seems to be an optimum range in the peak height and the number density of protrusions.
It has been found from various experiments conducted by the present inventor that by setting the peak height of the protrusions in a range of 0.05 to 0.50 xcexcm, and the number density in a range of 0.1 to 0.5 pc./xcexcm2, respectively, the protrusions and the conductive members never fail to come into contact with each other, and conductive connection between the first and the second transparent conductive films can be reliably secured.
Thus, a liquid crystal display device according to the present invention is further characterized in that the peak height of the protrusions is set in a range of 0.05 to 0.50 xcexcm, and the number density of the protrusions is set in a range of 0.1 to 0.5 pc./xcexcm2.
Further, to attain the first object, the present invention provides a method of manufacturing a liquid crystal display device, which comprises the steps of:
1) forming first and second transparent conductive films of predetermined pattern on opposed surfaces of first and second transparent substrates, respectively;
2) forming first and second transparent insulating films on portions of the first and second transparent conductive films corresponding to a liquid crystal display portion, respectively;
3) performing electroless plating using the first and second transparent insulating films as a mask to cause an alloy to selectively precipitate on portions of the first transparent and second transparent conductive films where the first and second transparent insulating films are not formed, thereby forming metal thin films on the portions of the first and second transparent conductive films;
4) printing a resin which contains second spacer members generally evenly dispersed therein, on one of the first and second transparent substrates, thereby forming a main seal portion;
5) printing a resin which contains first spacer members generally evenly dispersed therein, the first spacer members having a particle diameter smaller than that of the second spacer members by a predetermined value, on the other of the first and second transparent substrates, thereby forming a conducting seal portion;
6) laminating the first and second transparent substrates such that the main seal portion and the conducting seal portion are aligned with each other, to form a seal by the main seal portion and the conducting seal portion; and
7) injecting liquid crystal into the seal, and breaking the laminated first and second transparent substrates into a plurality of liquid crystal cells.
Further, to attain the second object, the present invention provides a method of manufacturing a liquid crystal display device, which comprises the steps of:
1) forming first and second transparent conductive films of predetermined pattern on opposed surfaces of first and second transparent substrates, respectively;
2) forming first and second transparent insulating films on portions of the first and second transparent conductive films corresponding to a liquid crystal display portion, respectively;
3) performing electroless Nixe2x80x94P alloy plating using the first and second. transparent insulating films as a mask to cause a NIxe2x80x94P alloy to selectively precipitate on portions of the first and second transparent conductive films where the first and second transparent insulating films are not formed, followed by electroless gold plating using a predetermined electroless gold plating bath to cause Au to precipitate on a Ni film by substitution reaction with P, thereby forming metal thin films with a multiplicity of protrusions having a predetermined peak height H and a number density xcfx81 formed on surfaces thereof on the portions of the first and second transparent conductive films;
4) printing a resin which contains second spacer members generally evenly dispersed therein, on one of the first and second transparent substrates, thereby forming a main seal portion;
5) printing a resin which contains first spacer members generally evenly dispersed therein, the first spacer members having a particle diameter smaller than that of the second spacer members by a predetermined value, on the other of the first and second transparent substrates, thereby forming a conducting seal portion;
6) laminating the first and second transparent substrates such that the main seal portion and the conducting seal portion are aligned with each other, to form a seal by the main seal portion and the conducting seal portion; and
7) injecting liquid crystal into the seal, and breaking the laminated first and second transparent substrates into a plurality of liquid crystal cells.
The above and other objects of the invention will become more apparent from the following description taken in conjunction with the accompanying drawings.